南郵專業(yè)英語報(bào)告-信號(hào)處理導(dǎo)論完整版(包含翻譯-原文和單詞)

南京郵電大學(xué)專業(yè)英語譯文報(bào)告學(xué)號(hào)學(xué)生姓名指導(dǎo)老師指導(dǎo)單位翻譯日期翻譯文獻(xiàn)IntroductiontoSignalProcessing譯文部分S.J.Orfanidis,IntroductiontoSignalProcessing,PrenticeHallInternational,Inc.,2003清華大學(xué)出版社有影印版，2003.7，中文書名：《信號(hào)處理導(dǎo)論》8.2數(shù)字音效象延時(shí)、回聲、回響、梳理濾波、（flanging）凸緣（法蘭）、合唱、pitchshifting(分聲步)、立體聲、變形、壓縮、擴(kuò)張、噪聲消除、均衡等等這樣一些音響效果在音樂制作和播放時(shí)是必不可少的。有些在家庭影院和汽車音響中已經(jīng)使用。大多數(shù)這樣的音響效果是用數(shù)字濾波器來實(shí)現(xiàn)的。這種數(shù)字濾波器也許是單獨(dú)的一個(gè)模塊，也可能是內(nèi)置在鍵盤或音調(diào)生成這樣的器件內(nèi)部。一般說來數(shù)字音效信號(hào)處理器如圖8.2.1所示。圖8.2.1數(shù)字音效信號(hào)處理數(shù)字音效處理器的輸入是由鍵盤或紀(jì)錄在其他介質(zhì)上的模擬信號(hào)，用一定的抽樣率抽樣。抽樣好的信號(hào)用DSP算法處理好以后再模擬重建輸出到下一級(jí)音頻通道中，如喇叭、混響器等等。全數(shù)字式系統(tǒng)可以不需要抽樣、重建部分，數(shù)字式輸入音頻信號(hào)可以一直在后續(xù)的DSP處理中保持?jǐn)?shù)字化。本屆中我們將討論一些基本音響效果，如延時(shí)、回聲、潤色、合唱、回響、動(dòng)態(tài)處理。具體的濾波器設(shè)計(jì)將在第十章和第十一章討論。8.2.1延時(shí)、回聲、梳理濾波程序chorus.m演示的是正弦信號(hào)經(jīng)合唱處理后的情形。調(diào)相(PhaseShifting)對(duì)吉他手、鍵盤演奏人員、歌唱家來說是經(jīng)常采用的一種效果。調(diào)相是把聲音信號(hào)用一個(gè)窄帶陷狀濾波器過濾，再把過濾信號(hào)的一部分與源信號(hào)相加而得到的。陷點(diǎn)的頻率以可控的方式調(diào)節(jié)，比如說可以用一個(gè)低頻振蕩器，也可以用腳踏板控制。陷點(diǎn)附近的頻率有較強(qiáng)的漂移，與原來的直接聲音結(jié)合，使得相位在頻率軸上發(fā)生抵消或加強(qiáng)，整個(gè)相位在頻率軸上出現(xiàn)波動(dòng)。一般說來，典型的單零點(diǎn)陷狀濾波器的幅頻響應(yīng)和相頻響應(yīng)如圖所示。notch.m。(seepage252forthereviewofnotchfilter)。注意到相頻響應(yīng)在相點(diǎn)處等于0，而在相點(diǎn)附近變化極快?！?.4.3中，我們討論了一種構(gòu)造陷狀濾波器的簡單方法，也就是相設(shè)計(jì)一個(gè)notch多項(xiàng)式N(z)，其零點(diǎn)就是我們要設(shè)計(jì)的陷點(diǎn)。然后再單位圓以內(nèi)靠外一點(diǎn)的相同頻率上設(shè)置濾波器的極點(diǎn)。這樣的濾波器的傳遞函數(shù)具有以下形式：這樣設(shè)計(jì)的濾波器可以構(gòu)造多陷點(diǎn)的相位漂移。選擇ρ接近等于1可以實(shí)現(xiàn)非常窄，但是這樣的濾波器不能夠?qū)Ω鱾€(gè)頻率陷點(diǎn)的相位單獨(dú)控制。用雙線性變換法(第十一章討論)設(shè)計(jì)的這種濾波器可以對(duì)陷點(diǎn)頻率和3-dB寬度進(jìn)行精確控制。這樣設(shè)計(jì)的濾波器單陷點(diǎn)濾波器的傳遞函數(shù)具有以下形式：(8.2.22)其中參數(shù)b用3-dB寬度Δω表示為：(8.2.23)衡量陷狀濾波器的另一個(gè)參數(shù)為品質(zhì)因數(shù)Q，用3-dB寬度表示為：(8.2.24)也就是說，品質(zhì)因數(shù)越高，陷點(diǎn)寬度越窄。因?yàn)樘幜讼蔹c(diǎn)以外幅頻響應(yīng)基本上不發(fā)生變化(Flat)，所以可以用多個(gè)這樣的濾波器級(jí)聯(lián)起來形成多陷點(diǎn)濾波器，各濾波器的陷點(diǎn)頻率和相位可以單獨(dú)調(diào)節(jié)。舉例來說，要設(shè)計(jì)一個(gè)陷點(diǎn)頻率為ω0=0.35π的陷頻濾波器，品質(zhì)因數(shù)分別為Q=3.5和Q=35兩種情況下，3-dB寬度為：和有(8.2.3)式計(jì)算得到濾波器系數(shù)和傳遞函數(shù)為：幅頻響應(yīng)和相位響應(yīng)如圖所示頻率漂移演示程序若陷點(diǎn)頻率隨時(shí)間變化，則3-dB寬度也會(huì)隨時(shí)間變化，濾波器的系數(shù)也是時(shí)間變化的。這樣的濾波器的時(shí)域?qū)崿F(xiàn)可以采用規(guī)范形式。比方說，如果陷頻是在ω1±ω2之間以ωSWEEP正弦變化，即ω0(n)=ω1+ω2sin(ωSWEEPn)，可以采用下列樣值處理算法來計(jì)算飄動(dòng)的濾波器系數(shù)，再分別計(jì)算每次輸入抽樣的濾波。Flanging、合唱、調(diào)相三種效果都是把一個(gè)簡單濾波器的系數(shù)設(shè)計(jì)尾隨輸入抽樣變化而使濾波器成為時(shí)變?yōu)V波器。自適應(yīng)信號(hào)處理也是隨時(shí)間改變?yōu)V波器的系數(shù)。系數(shù)與時(shí)間之間的關(guān)系是受某些設(shè)計(jì)條件的限制，即濾波器系數(shù)相對(duì)于輸入抽樣調(diào)節(jié)并且優(yōu)化。自適應(yīng)算法的實(shí)施也就是要求濾波器的樣值處理算法當(dāng)中考慮到隨輸入抽樣的不同系數(shù)有不同的權(quán)。自適應(yīng)濾波應(yīng)用范圍非常廣，象通道均衡、回聲消除、消噪聲、自適應(yīng)天線系統(tǒng)、自適應(yīng)喇叭均衡、自適應(yīng)系統(tǒng)辨識(shí)和控制、神經(jīng)網(wǎng)絡(luò)等等8.2.3回響回響回響的時(shí)間常數(shù)定義為房間的沖激響應(yīng)衰減到60dB的時(shí)間。一般的影院時(shí)間常數(shù)為1.8~2秒。電影院的聲音質(zhì)量取決于回聲沖激響應(yīng)，而沖激響應(yīng)主要是由聲源與觀眾的相對(duì)位置決定的。因此數(shù)字上模擬任何一個(gè)電影院回響特性幾乎是不可能的事。作為一種簡化，數(shù)字回響濾波器試圖模擬放映大廳具有特征性的回響沖激響應(yīng)，讓用戶有選擇性的調(diào)節(jié)某些參數(shù)，如前期反射的延時(shí)時(shí)間、或者是總體的回響時(shí)間。另一種有趣的回響效果是模擬濾波器無法完成的，這就是截?cái)郔IR響應(yīng)使其成為FIR而得到gatedreverb(選通回響)并且可以讓用戶調(diào)節(jié)截?cái)嗟臅r(shí)間。snaredrum(小鼓)的聲音就很適用于這樣處理。逆時(shí)間截?cái)嗟幕仨戫憫?yīng)在模擬領(lǐng)域是無法做到的。圖示的普通回響濾波器太簡單，難以產(chǎn)生實(shí)際的回響效果。Schroeder以依此為基礎(chǔ)來構(gòu)造復(fù)雜的回響器，這種濾波器可以由earlyreflection和latediffuse效果。大部分?jǐn)?shù)字信號(hào)處理中，我們感興趣的是穩(wěn)態(tài)響應(yīng)，而回響是例外，我們感興趣的是濾波器的暫態(tài)響應(yīng)，因?yàn)檎请娪霸旱臅簯B(tài)響應(yīng)才形成了回響效果。穩(wěn)態(tài)響應(yīng)決定了總體聲音質(zhì)量。普通回響濾波器穩(wěn)態(tài)頻譜的峰值加強(qiáng)了輸入信號(hào)峰值頻率附近的那些頻率。為了避免這種輸入聲音的加強(qiáng)程度不一致，Schroeder提出了一種全通濾波器，這種濾波器的幅頻響應(yīng)特性為一直線。濾波器的傳遞函數(shù)如下：(8.2.25)其I/O方程如下：(8.2.26)用z=ejω代入傳遞函數(shù)得到頻率響應(yīng)：(8.2.27)因?yàn)榉肿佣囗?xiàng)式和分母多項(xiàng)式的幅值相同，所以對(duì)所有頻率幅頻響應(yīng)為常數(shù)。盡管穩(wěn)態(tài)響應(yīng)為常數(shù)，濾波器的暫態(tài)響應(yīng)像普通的回響濾波器一樣指數(shù)衰減。事實(shí)上，將H(z)永部分分式展開得到：(8.2.28)其中，，把后面一項(xiàng)展開乘幾何級(jí)數(shù)得到：其沖激響應(yīng)為：(8.2.29)圖8.2.17是其框圖實(shí)現(xiàn)方法：普通回響器與全通回響器結(jié)合就可以形成實(shí)際的回響器。Schroeder的回響器就是用幾個(gè)普通回響單元并聯(lián)，后面在接上幾個(gè)級(jí)聯(lián)的全通濾波器組成的。(見本書封面上圖形和page372所示圖)。六個(gè)單元中不同的延時(shí)是回聲的強(qiáng)度增加，形成的沖激響應(yīng)具有典型的前期回聲和后期回聲效果。圖示為下列參數(shù)是回響器的沖激響應(yīng)。英文原文8.2DigitalAudioEffectsAudioeffects,suchasdelay,echo,reverberation,combfiltering,flanging,chorusing,pitchshifting,stereoimaging,distortion,compression,expansion,noisegating,andequalization,areindispensableinmusicproductionandperformance[115–151].Somearealsoavailableforhomeandcaraudiosystems.Mostoftheseeffectsareimplementedusingdigitalsignalprocessors,whichmayresideinseparatemodulesormaybebuiltintokeyboardworkstationsandtonegenerators.AtypicalaudioeffectssignalprocessorisshowninFig.8.2.1.Theprocessortakesinthe“dry”analoginput,producedbyaninstrumentsuchasakeyboardorpreviouslyrecordedonsomemedium,andsamplesitatanappropriateFig.8.2.1Audioeffectssignalprocessor.audiorate,suchas44.1kHz(orless,dependingontheeffect).ThesampledaudiosignalisthensubjectedtoaDSPeffectsalgorithmandtheresultingprocessedsignalisreconstructedintoanalogformandsentontothenextunitintheaudiochain,suchasaspeakersystem,arecordingchannel,amixer,oranothereffectsprocessor.Inall-digitalrecordingsystems,thesampling/reconstructionpartscanbeeliminatedandtheoriginalaudioinputcanremainindigitizedformthroughoutthesuccessiveprocessingstagesthatsubjectittovariousDSPeffectsormixitwithsimilarlyprocessedinputsfromotherrecordingtracks.Inthissection,wediscusssomebasiceffects,suchasdelays,echoes,flanging,chorusing,reverberation,anddynamicsprocessors.ThedesignofequalizationfilterswillbediscussedinChapters10and11.8.2.1Delays,Echoes,andCombFiltersPerhapsthemostbasicofalleffectsisthatoftimedelaybecauseitisusedasthebuildingblockofmorecomplicatedeffectssuchasreverb.Inalisteningspacesuchasaroomorconcerthall,thesoundwavesarrivingatourearsconsistofthedirectsoundfromthesoundsourceaswellasthewavesreflectedoffthewallsandobjectsintheroom,arrivingwithvariousamountsoftimedelayandattenuation.Repeatedmultiplereflectionsresultinthereverberationcharacteristicsofthelisteningspacethatweusuallyassociatewitharoom,hall,cathedral,andsoon.Asinglereflectionorechoofasignalcanbeimplementedbythefollowingfilter,whichaddstothedirectsignalanattenuatedanddelayedcopyofitself:y(n)=x(n)+ax(n?D)(echofilter)(8.2.1)ThedelayDrepresentstheround-triptraveltimefromthesourcetoareflectingwallandthecoefficientaisameasureofthereflectionandpropagationlosses,sothat|a|≤1.Thetransferfunctionandimpulseresponseofthisfilterare:H(z)=1+az?D,h(n)=δ(n)+aδ(n?D)(8.2.2)ItsblockdiagramrealizationisshowninFig.8.2.2.ThefrequencyresponseisobtainedfromEq.(8.2.2)bysettingz=ejω:（8.2.3）8.2.2Flanging,Chorusing,andPhasingThevalueofthedelayDinsamples,orinsecondsTD=DT,canhaveadrasticeffectontheperceivedsound[119,120,128].Forexample,ifthedelayisgreaterthanabout100millisecondsintheechoprocessor(8.2.1),thedelayedsignalcanbeheardasaquickrepetition,a“slap”.Ifthedelayislessthanabout10msec,theechoblendswiththedirectsoundandbecauseonlycertainfrequenciesareemphasizedbythecombfilter,theresultingsoundmayhaveahollowqualityinit.Delayscanalsobeusedtoalterthestereoimageofthesoundsourceandareindispensabletoolsinstereomixing.Forexample,adelayofafewmillisecondsappliedtooneofthespeakerscancauseshiftingandspreadingofthestereoimage.Similarly,amonosignalappliedtotwospeakerswithsuchasmalltimedelaywillbeperceivedinstereo.Moreinterestingaudioeffects,suchasflangingandchorusing,canbecreatedbyallowingthedelayDtovaryintime[119,120,128].Forexample,Eq.(8.2.1)maybereplacedby:(flangingprocessor)(8.2.17)Aflangingeffectcanbecreatedbyperiodicallyvaryingthedelayd(n)between0and10msecwithalowfrequencysuchas1Hz.Forexample,adelayvaryingsinusoidallybetweenthelimits0≤d(n)≤Dwillbe:(8.2.18)whereFdisalowfrequency,inunitsof[cycles/sample].ItsrealizationisshowninFig.8.2.8.Thepeaksofthefrequencyresponseoftheresultingtime-varyingcombfilter,occurringatmultiplesoffs/d,anditsnotchesatoddmultiplesoffs/2d,willsweepupanddownthefrequencyaxisresultinginthecharacteristicwhooshingtypesoundcalledflanging.Theparameteracontrolsthedepthofthenotches.Inunitsof[radians/sample],thenotchesoccuratoddmultiplesofπ/d.Intheearlydays,theflangingeffectwascreatedbyplayingthemusicpiecesimultaneouslythroughtwotapeplayersandalternatelyslowingdowneachtapebymanuallypressingtheflangeofthetapereel.Becausethevariabledelaydcantakenon-integervalueswithinitsrange0≤d≤D,theimplementationofEq.(8.2.17)requiresthecalculationoftheoutputx(n?d)ofadelaylineatsuchnon-integervalues.AswediscussedinSection8.1.3,thiscanbeaccomplishedeasilybytruncation,roundingorlinearinterpolation.Linearinterpolationisthemoreaccuratemethod,andcanbeimplementedwiththehelpofthefollowingroutinetapi.c,whichisageneralizationoftheroutinetaptonon-integervaluesofd.Theinputdmustalwaysberestrictedtotherange0≤d≤D.Notethatifdisoneoftheintegersd=0,1,...,D,theroutine’soutputisthesameastheoutputoftap.Themod-(D+1)operationinthedefinitionofjisrequiredtokeepjwithinthearraybounds0≤j≤D,andiseffectiveonlywhend=D,inwhichcasetheoutputisthecontentofthelastregisterofthetappeddelayline.Thefollowingroutinetapi2.cisageneralizationoftheroutinetap2,whichisimplementedintermsoftheoffsetindexqinsteadofthecircularpointerp,suchthatp=w+q./*tapi2.c-interpolatedtapoutputofadelayline*/Linearinterpolationshouldbeadequateforlow-frequencyinputs,havingmaximumfrequencymuchlessthantheNyquistfrequency.Forfastervaryinginputs,moreaccurateinterpolationmethodscanbeused,designedbythemethodsofChapter12.Asanexampleillustratingtheusageoftapi,considertheflangingofaplainsinusoidalsignaloffrequencyF=0.05cycles/samplewithlengthNtot=200samples,sothatthereareFNtot=10cyclesinthe200samples.Theflangedsignaliscomputedbywithd(n)givenbyEq.(8.2.18),D=20,andFd=0.01cycles/sample,sothatthereareFdNtot=2cyclesinthe200samples.Thefollowingprogramsegmentimplementsthecalculationoftheterms(n)=xandy(n).Adelay-linebufferofmaximaldimensionD+1=21wasused:double*w,*p;w=(double*)calloc(D+1,sizeof(double));p=w;for(n=0;n<Ntot;n++){d=0.5*D*(1-cos(2*pi*Fd*n));time-varyingdelayx=cos(2*pi*F*n);inputx(n)s=tapi(D,w,p,d);delay-lineoutputx(n?d)y=0.5*(x+s);filteroutput*p=x;delay-lineinputcdelay(D,w,&p);updatedelayline}Figure8.2.9showsthesignalsx(n),s(n)=xn?d(n),y(n),aswellasthetime-varyingdelayd(n)normalizedbyD.Recursiveversionsofflangerscanalsobeusedthatarebasedontheall-polecombfilter(8.2.13).ThefeedbackdelayDinFig.8.2.6isreplacednowbyavariabledelayd.TheresultingflangingeffecttendstobesomewhatmorepronouncedthanintheFIRcase,becausethesweepingcombpeaksaresharper,asseeninFig.8.2.7.Chorusingimitatestheeffectofagroupofmusiciansplayingthesamepiecesimultaneously.Themusiciansaremoreorlesssynchronizedwitheachother,exceptforsmallvariationsintheirstrengthandtiming.Thesevariationsproducethechoruseffect.AdigitalimplementationofchorusingisshowninFig.8.2.10,whichimitatesachorusofthreemusicians.Thesmallvariationsinthetimedelaysandamplitudescanbesimulatedbyvaryingthemslowlyandrandomly[119,120].Alow-frequencyrandomtimedelayd(n)intheinterval0≤d(n)≤Dmaybegeneratedby3588.SIGNALPROCESSINGAPPLICATIONSFig.8.2.10Choruseffect,withrandomlyvaryingdelaysandamplitudes.d(n)=D0.5+v(n)(8.2.20)or,ifthedelayistoberestrictedintheintervalD1≤d(n)<D2d(n)=D1+(D2?D1)0.5+v(n)(8.2.21)Thesignalv(n)isazero-meanlow-frequencyrandomsignalvaryingbetween[?0.5,0.5).ItcanbegeneratedbythelinearlyinterpolatedgeneratorroutineranlofAppendixB.2.GivenadesiredrateofvariationFrancycles/sampleforv(n),weobtaintheperiodDran=1/Franofthegeneratorranl.Asanexample,consideragainthesignaly(n)definedbyEq.(8.2.19),butwithd(n)varyingaccordingtoEq.(8.2.20).TheinputisthesamesinusoidoffrequencyF=0.05andlengthNtot=200.Thefrequencyoftherandomsignalv(n)wastakentobeFran=0.025cycles/sample,correspondingtoNtotFran=5randomvariationsinthe200samples.TheperiodoftheperiodicgeneratorranlwasDran=1/Fran=40samples.Thesameprogramsegmentapplieshere,butwiththechange:d=D*(0.5+ranl(Dran,u,&q,&iseed));wheretheroutineparametersu,q,iseedaredescribedinAppendixB.2.Figure8.2.11showsthesignalsx(n),s(n)=xn?d(n),y(n),aswellasthequantityd(n)/D.Phasingorphaseshiftingisapopulareffectamongguitarists,keyboardists,andvocalists.Itisproducedbypassingthesoundsignalthroughanarrownotchfilterandcombiningaproportionofthefilter’soutputwiththedirectsound.Thefrequencyofthenotchisthenvariedinacontrolledmanner,forexample,usingalow-frequencyoscillator,ormanuallywithafootcontrol.Thestrongphaseshiftsthatexistaroundthenotchfrequencycombinewiththephasesofthedirectsignalandcausephasecancellationsorenhancementsthatsweepupanddownthefrequencyaxis.AtypicaloverallrealizationofthiseffectisshowninFig.8.2.12.Multi-notchfilterscanalsobeused.Theeffectissimilartoflanging,exceptthatinflangingthesweepingnotchesareequallyspacedalongthefrequencyaxis,whereasinphasingthenotchescanbeunequallyspacedandindependentlycontrolled,intermsoftheirlocationandwidth.Themagnitudeandphaseresponsesofatypicalsingle-notchfilterareshowninFig.8.2.13.NotethatthephaseresponseargH(ω)remainsessentiallyzero,exceptinthevicinityofthenotchwhereithasrapidvariations.InSection6.4.3,wediscussedsimplemethodsofconstructingnotchfilters.ThebasicideawastostartwiththenotchpolynomialN(z),whosezerosareatthedesirednotchfrequencies,andplacepolesbehindthesezerosinsidetheunitcircle,atsomeradialdistanceρ.Theresultingpole/zeronotchfilterwasthenH(z)=N(z)/N(ρ?1z).Suchdesignsaresimpleandeffective,andcanbeusedtoconstructthemulti-notchfilterofaphaseshifter.Choosingρtobenearunitygivesverynarrownotches.However,wecannothavecompleteandseparatecontrolofthewidthsofthedifferentnotches. Adesignmethodthatgivesprecisecontroloverthenotchfrequencyandits3-dBwidthisthebilineartransformationmethod,tobediscussedindetailinChapter11.Usingthismethod,asecond-ordersingle-notchfiltercanbedesignedasfollows:(8.2.22)wherethefilterparameterbisexpressibleintermsofthe3-dBwidthΔω(inunitsofradianspersample)asfollows:(8.2.23)TheQ-factorofanotchfilterisanotherwayofexpressingthenarrownessofthefilter.Itisrelatedtothe3-dBwidthandnotchfrequencyby:(8.2.24)Thus,thehighertheQ,thenarrowerthenotch.Thetransferfunction(8.2.22)isnormalizedtounitygainatDC.ThebasicshapeofH(z)isthatofFig.8.2.13.Because|H(ω)|isessentiallyflatexceptinthevicinityofthenotch,severalsuchfilterscanbecascadedtogethertocreateamulti-notchfilter,withindependentlycontrollednotchesandwidths.Asanexample,considerthedesignofanotchfilterwithnotchfrequencyω0=0.35π,forthetwocasesofQ=3.5andQ=35.Thecorresponding3-dBwidthsareinthetwocases:和ThefiltercoefficientsarethencomputedfromEq.(8.2.23),givingthetransferfunctionsinthetwocases:Thesubjectofadaptivesignalprocessing[27]isalsobasedonfilterswithtimevaryingcoefficients.Thetimedependenceofthecoefficientsisdeterminedbycertaindesigncriteriathatforcethefiltertoadjustandoptimizeitselfwithrespecttoitsinputs.Theimplementationofanadaptivealgorithmisobtainedbyaugmentingthesampleprocessingalgorithmofthefilterbyaddingtoitthepartthatadjuststhefilterweightsfromonetimeinstanttothenext[28].Adaptivesignalprocessinghaswidespreadapplications,suchaschannelequalization,echocancellation,noisecancellation,adaptiveantennasystems,adaptiveloudspeakerequalization,adaptivesystemidentificationandcontrol,neuralnetworks,andmanyothers.8.2.3DigitalReverberationThereverberationofalisteningspaceistypicallycharacterizedbythreedistincttimeperiods:thedirectsound,theearlyreflections,andthelatereflections[115–151],asillustratedinFig.8.2.15.Thesoundqualityofaconcerthalldependsonthedetailsofitsreverberationimpulseresponse,whichdependsontherelativelocationsofthesoundsourceandthelistener.Therefore,simulatingdigitallythereverbcharacteristicsofanygivenhallisanalmostimpossibletask.Asacompromise,digitalreverbprocessorsattempttosimulateatypicalreverberationimpulseresponseofahall,andgivetheusertheoptionoftweakingsomeoftheparameters,suchasthedurationoftheearlyreflections(thepredelaytime),ortheoverallreverberationtime.Otherinterestingreverbeffectscanbeaccomplisheddigitallythataredifficultorimpossibletodobyanalogmeans.Forexample,gatedreverbisobtainedbytruncatingtheIIRresponsetoanFIRone,asshowninFig.8.2.16,withauser-selectablegatetime.TheplainreverbfiltershowninFig.8.2.6istoosimpletoproducearealisticreverberationresponse.However,assuggestedbySchroeder[143],itcanbeusedasthebuildingblockofmorerealisticreverbprocessorsthatexhibitthediscreteearlyreflectionsandthediffuselateones.InmostapplicationsofDSP,weareinterestedinthesteadystateresponseofourfilters.Reverberationisanexception.Here,itisthetransientresponseofahallthatgivesititsparticularreverberationcharacteristics.Thesteady-stateproperties,however,dohaveaneffectontheoverallperceivedsound.Thepeaksinthesteady-statespectrumoftheplainreverbfilterofEq.(8.2.12),showninFig.8.2.7,tendtoaccentuatethosefrequenciesoftheinputsignalthatarenearthepeakfrequencies.Topreventsuchcolorationoftheinputsound,Schroederalsoproposed[143]anallpassversionoftheplainreverberatorthathasaflatmagnituderesponseforallfrequencies:(8.2.25)IthasI/Odifferenceequation:(8.2.26)Itsfrequencyandmagnituderesponsesareobtainedbysettingz=ejω:(8.2.27)ThemagnituderesponseisconstantinωbecausethenumeratoranddenominatorofH(ω)havethesamemagnitude,ascanbeseenfromthesimpleidentity:Figure8.2.17showsthecanonicalrealizationofEq.(8.2.25)realizedbyacommondelayz?D.ItalsoshowstheparallelrealizationofEq.(8.2.28),whichwasSchroeder’soriginalrealization[143].Theplainandallpassreverberatorunitscanbecombinedtoformmorerealisticreverbprocessors.Schroeder’sreverberator[143,115,119,137,127,139]consistsofseveralplainunitsconnectedinparallel,whicharefollowedbyallpassunitsincascade,asshowninFig.8.2.18.Theinputsignalcanalsohaveadirectconnectiontotheoutput,butthisisnotshowninthefigure.專業(yè)名詞術(shù)語總結(jié)flanging凸緣compression壓縮equalization均衡instrument儀器timedelay延時(shí)linearinterpolation線性插值relativesidelobelevel相對(duì)旁瓣水平physicalfrequencyresolution物理頻率分辨率computationalfrequencyresolution計(jì)算頻率分辨率resolvabilitycondition可分辨條件computationaloverhead額外的計(jì)算開銷PhaseShifting調(diào)相Narrownotchfilter窄帶陷狀濾波器Directsound源信號(hào)Single-notchfilter單陷點(diǎn)濾波器Magnitudesquared幅頻響應(yīng)Phaseresponses相位響應(yīng)prototype原型linearphase線性相位guaranteesability保證穩(wěn)定性lowpass低通highpass高通bandpass帶通bandstop帶阻transitionband過渡帶passband通帶zeropadding補(bǔ)零biasingerror偏移誤差roundingerror舍入誤差matrixform矩陣形式twiddlefactor旋轉(zhuǎn)因子modulo-N模NFFT(fastFouriertransform)快速傅立葉變換shuffling重排bitreversal碼位倒置fastconvolution快速卷積zero-meanwhiteGaussiannoise零均值高斯白噪聲minimizing最小化attenuation衰減transferfunction傳遞函數(shù)impulse沖激alter改變maximizing最大化piece-wiselinear分段線性time-windowing時(shí)域加窗finite-duration有限長samplingrate采樣率samplingtimeinterval采樣間隔rectangularwindow矩形窗hammingwindow漢明窗windowfunction窗函數(shù)frequencyleakage頻率泄露mainlobe主瓣sidelobe旁瓣mainlobewidth主瓣寬度relativesidelobelevel相對(duì)旁瓣水平physicalfrequencyresolution物理頻率分辨率computationalfrequencyresolution計(jì)算頻率分辨率resolvabilitycondition可分辨條件computationaloverhead額外的計(jì)算開銷exponentiallydecayingsinusoid包絡(luò)按指數(shù)衰減的正弦波wavetablesynthesis波表合成periodicsequence周期序列periodicwaveformgenerator周期波形產(chǎn)生器